04:37:05 UT, 2000 frames/second

Figure 5.10 shows a high-speed video sequence obtained at 2000 frames/second of a cluster of carrot and columniform sprites. The grey-scale was stretched somewhat so that fainter features would be more visible. The charge moment change of the parent +CG discharge was determined from magnetic-field ELF measurements by a technique identical to that used in Chapter 4 (see Section 4.1). The NLDN detected the +CG, but could not give an accurate estimate of its location or peak-current magnitude.

Figure 5.10: The high-speed video sequence of the 04:37:05 UT sprite cluster is correlated with the charge moment change following the parent +CG. The horizontal lines correspond to conventional breakdown thresholds between 70-80 km MSL altitude, assuming a uniformly-charged disk with $Z_d\,=\,$7.5 km and $R_d\,=\,$15 km (see Section 2.5.2) and an ionosphere conducting ledge at 81 km MSL. A sprite began in frame a at $\simeq$330 C$\cdot$km. The sprite cluster produced a significant charge moment change, as shown by the separation between the total charge moment change (thick line) and the parent discharge charge moment change (thin line). The charge moment curve was provided by Steve Cummer.

The approximate light integration periods of frames $a$-$f$ are bounded by vertical dashed lines in the charge moment plot of Figure 5.10. As discussed in Appendix A.1, the high-speed video frame is integrated in horizontal blocks. The light integration periods correspond to that of the 3rd block down from the top, since this is the block in which the sprites first appeared in frames $a$ and $b$.

A single sprite column is visible in frame $a$, just over 5 ms after the onset of the +CG. The +CG's cumulative charge moment change was 310-350 C$\cdot$km during frame a. Additional sprite columns appeared in frame $b$. It was shown in Section 5.2.3 that the onset of sprites can be at the limit of detectability at $\sim\,$300 km range. It follows that sprite onset may have been undetected at the $\sim\,$900 km range to this sprite cluster. As Figure 5.2 demonstrated, sprites usually brightened dramatically in the initial frames, so the initiation of these sprites probably occurred not much more than one frame earlier than the first detected luminosity. Thus, the charge moment threshold for sprite initiation was $\simeq\,$300 C$\cdot$km. A similar threshold was observed for other sprite clusters in this study.

A 300 C$\cdot$km sprite-initiation threshold is consistent the 200-400 C$\cdot$km estimate based on static-field measurements presented in Chapter 5. This threshold is also consistent with earlier ELF measurements of the minimum charge moment change associated with sprite-producing discharges (Cummer and Inan, 1997; Huang et al., 1999). Based on an assumption of a breakdown threshold of 100 Td, Fernsler and Rowland (1996) predicted that 300 C$\cdot$km would be sufficient to initiate conventional breakdown at 80 km altitude. The threshold at 80 km calculated in this work (see Figure 2.9) is somewhat larger than 300 C$\cdot$km since a value of 123 Td was used for the breakdown threshold, as discussed in Section 2.2.3.

In Section 5.2.4, it was shown that sprites typically initiated at 76-79 km MSL altitude. Based on a uniformly charged disc model presented in Section 2.5.2 with $R_d=15$ km and $Z_d=7.5$ km MSL, corresponding to measurements presented in Chapter 3, the charge moment threshold for conventional breakdown is $\simeq$430 C$\cdot$km at 79 km MSL and $\simeq$680 C$\cdot$km at 76 km MSL. Thus, the measured charge moments for sprite initiation were 1.4-2.3 times less than required to initiate conventional breakdown in a homogeneously stratified atmosphere. Significant spatial variation in conductivity might enhance the electric field in localized regions to the point of breakdown. Such conductivity inhomogeneities might be produced by radiated fields from the lightning discharge (Valdivia et al., 1998).